Many diseases are caused by cell-specific (e.g., tissue specific, organ specific, etc.) problems in the patient. Oftentimes, drugs and other treatment agents that act to counter diseases in these cells, tissues, and organs that are associated with the disease also cause unwanted side effects due to their actions in other cell and tissue types or due to general systemic activity in the patient.
For instance, aldosterone is a mineralocorticoid receptor agonist that promotes inflammation and cardiovascular disease. Mineralocorticoid receptor antagonists provide one of the few therapeutic approaches to treat heart failure and reduced left ventricular ejection fraction. The mineralocorticoid receptor antagonists spironolactone and eplerenone inhibit the mineralocorticoid receptor driven inflammatory modules and are effective in inhibiting sodium reabsorption and decreasing blood pressure in the cardiovascular system, thus aiding the treatment of cardiovascular disease. However, despite these clinical benefits on the heart and blood vessels, traditional mineralocorticoid receptor antagonists also produce unwanted side effects such as gynecomastia and mastodynia in men due to off-target effects on steroid receptors and hyperkalemia due to off-target actions in the kidney. As another example, conventional chemotherapies (e.g., for cancer) are administered systemically. While killing of only the cancer cells (e.g., of a tumor) is desirable, systemic administration exposes healthy, non-cancerous cells to these powerful and toxic chemotherapeutic agents, often producing sickness and other symptoms such as hair loss.
Thus, there is a need for agents that direct small molecules (e.g., drugs and other therapeutic agents) to specific cells, tissues, or organs for treatment, thereby minimizing or preventing the delivery of these small molecules to other cells, tissues, and organs and, consequently, reduces or eliminates off-target and generalized side effects.